A variety of high temperature processes are known which employ monolithic catalyst structures to promote the desired reactions, for example, partial oxidation of hydrocarbons, complete oxidation of hydrocarbons for emissions control, catalytic mufflers in automotive emissions control and catalytic combustion of fuels for further use in gas turbines, furnaces and the like. Typical of such catalytic systems are the catalysts used in thermal combustion units for gas turbines to provide low emissions and high combustion efficiency. To achieve high turbine efficiency, a high gas temperature is typically required. This, of course, places a high thermal stress on the catalyst monolith employed. An example of a monolithic catalyst structure is a unitary or bonded metallic or ceramic structure made up of a multitude of longitudinally disposed channels for passage of the combustion gas mixture. At least a portion of the channels are coated on their internal surfaces with a combustion catalyst.
In addition to high thermal stress, the high gas flow rates characteristic of combustion units in gas turbines place a significant axial load or force on the catalyst structure pushing in the direction of the gas flow due to the resistance to gas flow, i.e., friction, in the longitudinally disposed channels of the catalyst structure. For example, if a multistage monolithic catalyst structure such as that described in U.S. Pat. No. 5,183,401 to Dalla Betta et al. is employed as a 20 inch diameter catalyst in a catalytic combustion reactor where the air/fuel mixture flow rate is about 50 lbs/second at a pressure drop through the catalyst of 4 psi, the total axial load on the catalyst would be about 1,260 lbs.
The combination of exposure to both high temperatures, e.g., temperatures approaching and even exceeding 1,000.degree. C., where metallic monoliths begin to lose strength, and the aforesaid large axial loads (from high gas flow rates) can cause significant movement or deformation of the catalyst support in the direction of the gas flow. In fact, in cases where a corrugated metal foil catalyst monolith is used in which the corrugated foil is rolled together in a non-nesting fashion to form a cylindrical, spiral structure in which the foil layers are not bonded together, the combined high temperature and large axial load from high gas flow can cause the whole structure to telescope in the direction of gas flow, particularly when the axial force exceeds the foil-to-foil sliding resistance in the wound structure. Thus, there is a need to provide a support for the catalyst structure to secure it from movement and/or deformation along its axis in the direction of gas flow by means of a support structure which will provide the necessary support at high temperatures preferably without interfering with the efficiency and effectiveness of catalytic combustion as a source of motive force for a gas turbine.
Monolithic catalyst structures can be supported by positioning struts or bars adjacent the outlet end of the catalyst structure. In U.S. Pat. No. 5,461,864 to Dalla Betta et al., the use of internally cooled support struts or bars at the outlet of the catalyst structure is described as a means to support the catalyst. This approach, however, has the disadvantage that the support struts require a source of cooling air and this results in a more complicated combustion system design or requires the use of high pressure air that may not be available in the gas turbine machine. Also, since the air cooled struts are rather widely spaced over the face of the catalyst, high local contact forces or stresses can result. In certain portions of the catalyst design, these contact forces can exceed the yield strength of the relatively thin foil of the catalyst structure resulting in deformation of the foil. This would clearly not be a desirable result and would detract from usage of the air-cooled support struts in high axial load applications.
To overcome the disadvantage of the internally cooled support struts or bars, a monolithic honeycomb or monolithic open cellular support structure can be used to support the monolithic catalyst structure. Copending U.S. patent application Ser. No. 08/462,639 to Dalla Betta et al. Filed on Jun. 5, 1995 describes such a monolithic open support structure. The support structure includes a multiplicity of longitudinally disposed parallel channels positioned adjacent one another, similar to a honeycomb structure. The support structure is formed by relatively thin strips or ribs of high temperature resistant metal or ceramic material which are bonded together to form a unitary structure. The support structure abuts against and extends over the entire outlet face of the catalyst structure. The channels provide for passage of the flowing gas mixture through the support structure. The peripheral edge of the support structure is secured to the reaction chamber wall so that the axial load acting on the support structure is transferred to the reaction chamber wall, for example, such as by a radially inwardly extending ridge formed on the reaction chamber wall.
It has been discovered that during operation of the gas turbine reactor, the high temperatures generated therein cause the relatively thin walled monolithic open cellular support structure and catalyst structure to thermally expand by a significantly greater magnitude than the relatively thick walled reaction chamber wall. To overcome this problem and avoid crushing or deformation of the catalyst and support structures, the support structure and the catalyst structure should be sized so that their outside diameters are smaller than the inside diameter of the reaction chamber wall to allow thermal expansion of the support structure during such high temperature operation. If the outside diameter of the support structure is too large, the support structure may deform or bulge outwardly against the catalyst structure resulting in significant damage to the foils of the catalyst structure.
During operation of the reactor, the temperatures of the gas mixture, the catalyst structure, the support structure, and the reaction chamber wall can vary, resulting in different rates of thermal expansion for the catalyst structure, the support structure, and the reaction chamber wall. For example, the reactor can have a preburner located upstream of the catalyst. The preburner is used to start the reactor and provide a required catalyst inlet temperature. Initially, fuel is added to the preburner to give a high temperature gas which flows through the catalyst and to the turbine to start the engine. Since the preburner can respond relatively quickly, the temperature of the gas exiting the preburner and flowing into the catalyst structure rises relatively rapidly. The catalyst substrate has a relatively low heat capacity and also increases in temperature quickly. Similarly, the support structure increases in temperature relatively quickly due to the relatively thin metal monolithic structure. The rapid rise in temperature of the catalyst structure and the support structure causes rapid thermal expansion therein. However, due to the thickness of the reaction chamber wall and exposure to cooler air on the outside thereof, the reaction chamber wall heats up at a lower rate, and thermally expands at a slower rate than the catalyst structure and the support structure.
To avoid the deformation and crushing of the catalyst structure and the support structure, the dimensions of the outside diameter of the catalyst and support structure and the catalyst structure, and the inner diameter of the reaction chamber wall can be sized so that there would be substantially no gap therebetween during the initial heating of the reactor. However, once the reactor reaches an idle condition and the reaction chamber wall rises in temperature, a gap would exist between the reaction chamber wall and the catalyst and support structures. After idle condition, the preburner is turned down and the catalyst structure and the support structure would cool and thermally contract away from the reaction chamber wall, thereby increasing the space therebetween.
Although the outside diameter of the catalyst and support structure and the catalyst structure can be reduced to compensate for the thermal expansion, this results in a relatively large annular gap or space between the inside diameter of the reaction chamber wall and with the outside diameters of the catalyst and support structures during various cycles of the gas turbine. Due to the relatively large annular space, a significant amount of gas mixture can bypass the catalyst structure resulting in non-uniform temperatures and gas compositions at the outlet of the catalyst structures. This, in turn, can produce significantly high undesirable emissions, such as Carbon Monoxide (CO) and Unburned Hydrocarbons (UHC) and/or result in higher combustion temperatures in the homogeneous combustion zone downstream of the catalyst, thus producing higher nitrogen oxide (NO.sub.x) emissions. It has been discovered that the positioning of the support structure against the radially inwardly extending ridge formed on the reaction chamber may not provide an adequate seal therebetween and can allow an unacceptable amount of gas flow through the annular space.
It has also been found that the catalyst structure and the support structure can thermally expand in a lateral direction in a non-uniform manner, especially if the support structure possesses a different coefficient of thermal expansion than the catalyst structure. The lateral movement can cause the outlet face of the catalyst structure to scrape against the inlet face of the support structure resulting in damage to the catalyst foils.